Conditional Processing with NoPass Parameters

["Bill" <billzbubb@xxxxxxxxxx>]

I have a program where I'd like one of the parameters to be optional (to 
maintain compatiblity with a prior version of the program).  This is the 
current prototype of the new program:

D CalcDat         pr                  ExtPgm('CALCDAT')          
D  inpDat                       10                               
D  inpDatFmt                    10                               
D  inpDays                       4  0                            
D  outDat                       10                               
D  outDatFmt                    10                               

I'd like to make outDatFmt optional since the original didn't have that 
parameter.  I believe this is done via the "Options(*NoPass)", correct?  If so, 
I believe if I use *NoPass I can't change the value of outDatFmt like this:
                                       
      If outDatFmt = *Blanks ;         
        outDatFmt = inpDatFmt ;        
      EndIf ;                          
                                       
So, my question is:  What would be the best way to accomplish the above goals?  
Something along the lines of a working variable for outDatFmt?  Are there other 
ways of checking for the missing parameter than "If %Parms" ?

Bill